1. Field of the Invention
The present invention relates to an apparatus for manufacturing long-period fiber gratings, and to an apparatus for manufacturing two-band long-period fiber gratings.
2. Description of the Related Art
A long-period fiber grating couples a fundamental core mode of an optical fiber to the cladding lead modes. This type of fiber grating has an advantage over the reflection type of grating of flattening the gain of a fiber amplifier.
Long-period fiber gratings are typically manufactured by periodically varying along an optical fiber the refractive index of a UV-sensitive core of the optical fiber by exposure to UV rays. In other words, the portion of the optical fiber core exposed to UV rays exhibits an increase in the refractive index and the non-exposed portion exhibits no change in the refractive index, so that a periodic change in the refractive index occurs. Coupling will occur in the core of an optical fiber under the condition expressed in Equation (1): ##EQU1##
where .beta..sub.co is the propagation constant of a core mode, .beta..sub.cl.sup.n is the propagation constant of a cladding mode and .LAMBDA. is the grating period of the optical fiber.
When .beta. is substituted with ##EQU2##
where, n is the effective refractive index, Equation (1) can be rewritten stating that the difference between refractive indices of the core mode and the cladding mode, (n.sub.co -n.sub.cl.sub.n), is equal to ##EQU3##
Therefore, coupling wavelength .lambda. depends on the grating period .LAMBDA. and the refractive index difference n.sub.co -n.sub.cl.sup.n. For a fixed grating period .LAMBDA., coupling wavelength .lambda. changes with n.sub.co -n.sub.cl.sup.n, the refractive index difference which can be obtained by appropriately irradiating UV laser into the optical fiber which is sensitive to UV rays.
If a CV laser irradiates a photosensitive optical fiber, the refractive index of the core of the optical fiber increases and as a result, the coupling occurs at a longer wavelength. A long-period fiber grating is manufactured by focusing UV laser such as an excimer laser in the x- or y-axis using a cylindrical lens and irradiating the same into a photosensitive optical fiber through an amplitude mask having a periodicity .LAMBDA.. In the case of an excimer laser with a 10.times.30 mm beam size, the beam is largest when it is focused to a rectangular axis of 30 mm long excimer laser. That is, in this case, the maximum length of the beam along one axis is 30 mm.
Here, it is important for the amplitude mask to have an accurate periodicity. In order to attain an accurate periodicity of the amplitude mask, various methods are employed. One of them is a method in which a single slit or optical fiber is installed in a translation stage and is then shifted by a desired period between steps of irradiating laser light thereon. The single slit method has advantages in that the periodicity of the slit is accurate and arbitrarily adjusted. However, according to this method, since the width of the slit is fixed, the duty cycle, which is a ratio of a transmitted domain of light and a non-transmitted domain of light, is not constant when the period of a long-period fiber grating is changed. Also, since the refractive index change is obtained point by point using a slit, much time is required and much of the beam of laser light cannot be effectively utilized. Further, in order to accurately design a desired filter spectrum, it is necessary to know the precise change in refractive index achieved per pulse. Moreover, an expensive translation stage is necessary.
Alternatively, in order to make the periodicity of a mask accurate, the mask can be manufactured by patterning with chrome on silica. However, according to this method, the mask manufacturing process is complicated and costly. Also, since the period of the mask is fixed, only a single spectrum can be created using a particular mask. Further, in this case, since the damage threshold power for this type of mask is low, the high power of an excimer laser cannot be effectively used.
Another method is to use multiple slits. The process for preparing a mask is simple and the costs therefor are low. However, since an error of one mask periodicity caused during laser treatment is very large, that is, .+-.5 .mu.m, an accurate spectrum design cannot be achieved easily. Also, since the period is fixed, the spectrums which can be designed are limited.
In general, the gain curve of an erbium-doped fiber amplifier (EDFA) can be flattened when two or three different long-period fiber gratings are used, which means two or three different mask periods are necessary. Because the gain curve depends on the input signal light power, pump power, rare earth dopant rate, glass matrix, fiber length and so on, as many mask periods as possible are necessary.